The mounting of discrete electrical components onto printed circuit boards by automated means is well known. For many years the mounting of such components having leads has been accomplished by hand. This is a tedious time-consuming and costly procedure which is especially true where miniature diodes, capacitors, resistors and transistors are concerned. Over the years, several types of apparatus has been developed to automate this procedure and thereby increase production. Basically, these machines align the components and their leads with respect to lead holes in the printed circuit board to be populated, and then insert the leads into the appropriate holes. The leads are then bent over or clinched to retain the components on the board prior to a soldering operation which both mechanically and electrically bonds the component to the board. In more recent times, however, a new electrical component packaging technique has been developed which does not require through-hole insertion. These are known as surface mounted components (SMC).
Surface mounted components lack or eliminate the long leads associated with through-hole mounted components. SMC's are finding wide use in the electronic industry, and like their leaded counterparts, require accurate placement on printed circuit boards, or other substrate media, prior to soldering. Accordingly, automatic machines have been developed to accomplish this task.
In an early SMC-delivering machine, the components are fed via tubes and apertures and are positioned on a slidable plate, situated over a printed circuit board, in a position which corresponds to the desired position of the components on the board. Subsequently, the plate is displaced while the components are retained so that the components ultimately drop onto the substrate. Due to the uncontrolled dropping motion, there is a risk that the components will tilt, rotate, or land in an incorrect position on the board.
In U.S. patent application No. 4,393,579 a device for mounting chip-type electronic components on a substrate is disclosed which is designed to enable accurate positioning and mounting of chip-type components in a controlled and reproducible manner. This device includes a slide having a recess for accommodating and transporting a component. The component is linearly displaceable in a guide between a loading position and an unloading position, and a suction device which is displaceable in the vertical direction serves to pick up a component from the slide when the latter is in the unloading position, and then move the component downward through a passage in the guide to position the picked up component onto the substrate positioned underneath the guide. Although this is an improvement over the previously known art, it still leaves much to be desired in speed of operation and reliability.
Also according to the prior art according to U.S. patent application No. 4,381,601, there is disclosed a method for mounting electronic components such as semiconductor pellets to the tip end of an attraction nozzle, holding and automatically orienting the sucked electronic component by means of a plurality of arms arranged along the outer periphery of the tip end of the sucking nozzle, transferring the held electronic component to a predetermined bonding position on a semiconductor device substrate, and opening the arms to bring down the electronic component onto the predetermined bonding position for bonding. Although this apparatus is adequate for the task of picking up and depositing a component, it does not include a fast and reliable means for transporting the component from an input station to an output station.
Stated in another way, the prior art is typified by transport means where components are positioned in a single row or on a semicircle so that the exposed SMC is presented at a consistent pick up point. A pick up device moves to the next required SMC and typically using a vacuum stylus, removes the SMC from its cavity. The pick up device then moves the single SMC to a predetermined location above the printed circuit board, placing the SMC thereon. This fetch and place cycle is repeated until the board is populated. Due to the pick up device speed, this prior art concept limits machines to 2,000 to 3,000 parts per hour.
In medium throughput (5,000 to 7,000 parts per hour) prior art machines, the SMC pick up device is commonly configured so as to move it a small linear distance, just in clearance of the reel, prior to placing a single SMC on the printed circuit board. As the pick up location is fixed, the desired reel must be moved to this location for SMC removal from the appropriate carrier cavity. The reels are typically moved and positioned with a precision lead screw using a servo drive or, in one design, a carousel. By careful programming, the reel-to-reel index time can be minimized, but generally any long reel index will pace the placement cycle speed.
Prior art high speed (9,000 to 15,000 parts per hour) machines place one component at a time, usually incorporating a sequencer to supply SMC to the placement head. As the name implies, sequencers stage (queue) up SMC in their proper order prior to actually being needed for placement. As SMC are being placed on a printed circuit board, another queue of SMC is being developed for the next board in order to keep the speed high. Occasionally, an individual SMC may be dropped or "lost" causing a gap in the queue which, in turn, results in a missed location on the board. It is clear that 100 percent populated printed circuit boards are required to justify the purchase of automatic assembly machines, so "repair" of the missing SMC should be accomplished when discovered.
The latter type high volume machines now generally incorporate means for "repairing" missed SMC by removing the printed circuit board in question, finishing the next board placement sequence, and returning the questionable board for repair. Obviously, this greatly increases handling time (non-placement time), lowers the net placement speed. It should, therefore, be evident that a relatively simple yet reliable random access electrical component mounting system which obviated the aforementioned shortcomings would constitute a significant advancement in the art.